1. Field of the Invention
The present invention relates to a device for sensing a weight of food products in a microwave oven, and more particularly to a food weight sensing device utilizing a variation in capacitance depending on a variation in overlapping area between electrode plates.
2. Description of the Prior Art
As conventional weight sensing methods, there have been known a method utilizing a capacitance, a method utilizing a differential transformer and a method utilizing piezoelectric elements and a method utilizing a strain gauge.
From these methods, the method utilizing the capacitance is mainly applied to microwave ovens. For measuring the weight of an object (namely, the food), this method utilizes the following equation (1): EQU C=.epsilon.A/d (1)
wherein, C represents a capacitance, .epsilon. a dielectric constant, A the area of electrode plates, and d the distance between the electrode plates.
A process of measuring the weight of an object by using the equation (1) is carried out as follows.
When an object is laid on a tray of a microwave oven, it applies its weight to the tray, thereby causing the distance d between electrode plates expressed by the equation (1) to be varied. Due to such a variation, the capacitance C is varied.
Microwave ovens measure the weight of an object, using the above-mentioned principle.
Now, a conventional capacitance type weight sensing device using the above-mentioned principle will be described, in conjunction with FIG. 1.
FIG. i is a sectional view of the capacitance type weight sensing device adapted to a conventional microwave oven. As shown in FIG. 1, the device comprises a base plate 101 to which a L-shaped bracket 102 is fixedly mounted. A fixed spacer 103 of a 90.degree.-turned U shape is fixedly mounted to the upper portion of the bracket 102. To the fixed spacer 103, a movable spacer 105 of a 270.degree.-turned U shape is connected by means of upper and lower parallel plate springs 104a and 104b, so as to face the fixed spacer 103.
The plate springs 104a and 104b serve as cantilevers permitting the movable spacer 105 to move vertically within a predetermined distance, with respect to the fixed spacer 103, that is, the fixed support.
To the movable spacer 105 is fixedly mounted a load bracket 106 which has an inverted L shape. A tray 108 is mounted to the upper portion of the load bracket 106.
Beneath the fixed spacer and movable spacer 103 and 105, a pair of electrode plates 107a and 107b defining a predetermined space therebetween are mounted to the fixed spacer and movable spacer 103 and 105, by means of proper support members, respectively.
Operation of the device with the above-mentioned construction will now be described.
As an object is laid on the tray 108, the plate springs 104a and 104b fixed to the fixed spacer 103 in a cantilever manner are downwardly bent due to the weight of object such that their free ends spaced away from the fixed spacer 103 move downwardly. Accordingly, the electrode plate 107b fixed to the movable spacer 105 which is fixed to the free ends of the plate springs 104a and 104b moves downwardly, so that it moves away from the electrode plate 107a. As a result, the distance between the electrode plates 107a and 107b varies, thereby causing the capacitance C to vary.
Thus, the weight of object can be measured, using such a variation in capacitance.
However, the above-mentioned conventional capacitance type weight sensing device involves the following problems.
First, the variation of the distance d between electrode plates caused by the weight of an object to be measured is achieved within a narrow range, thereby causing the variation in capacitance to be also achieved within a narrow range. As a result, it is difficult to achieve a precise measurement.
Second, overall size of the device is undesirably large, since the electrode plates 107a and 107b have a large size of, for example, 100 mm.times.100 mm.
Third, since the plate electrodes 107a and 107b are exposed to external, there is a problem of an error in measurement caused by variations in environmental factors such as dust, temperature and humidity.